1. Field of the Invention
This invention relates generally to the use of magnetic sensors for determining the absolute position of a target having magnetic poles and, more particularly, to the use of multiple arrays, each having a plurality of magnetic sensors to determine the absolute position of the target.
2. Related Prior Art
In determining position of a target, multiple magnetic sensors have been placed in proximity to a multi-pole magnet. Previously, strings of Hall effect devices have been used to provide signals that are summed to produce a sine wave that is representative of the average magnetic field over the multi-pole magnet. Summing the signals from the sensor elements in the first half of the Hall string and subtracting the sum of the signals from the sensor elements in the second half of the Hall string produces a cosine wave. The sine/and cosine waves can then be used to determine the position of a target equipped with the multi-pole magnet.
Such devices are useful in reducing errors in determining position of the target that are due to variations in the magnetic fields or the spacing of the magnetic poles. However, these known devices and methods have requirements on the string of sensor devices. The requirement is that they have a length that is matched precisely to the pole spacing of the multi-pole magnet with which the magnetic sensors are to be used. In addition, errors can be caused by external magnetic fields. The external fields will bias the magnetic field by increasing the magnetic North poles and decreasing the magnetic South poles or visa versa. This will cause the sine and cosine curves to shift such that the zero points will not be regularly spaced. In either case, the signal processing of the sine and cosine waves produces errors in the position measurement.
Several United States patents have been issued illustrating the current art, for example, U.S. Pat. No. 5,029,304, titled xe2x80x9cSensor With Absolute Digital Output Utilizing Hall Effect Devicesxe2x80x9d, issued to Robert J. Tolmie, Jr., et al. relates to a sensor for determining position or dimensions of an object that has an array of detectors and an actuating medium of the detectors configured to cause the detectors to output an absolute Gray binary code.
U.S. Pat. No. 5,574,445, titled xe2x80x9cDigital Absolute Position Encodersxe2x80x9d, issued to Robert L. Maresca, et al., relates to a position encoder for determining the absolute position of a first member with respect to a second member. The absolute position is determined within a resolution cell of L/2n where n is an integer and L is a predetermined span. One of the members has a plurality of m tracks of binary indicia distributed over the predetermined span, L, where m is an integer less than n. The other member has a plurality of indicia detectors disposed on the second member for detecting the binary indicia. With such an arrangement, an encoder is provided having xe2x80x9ca unit-distance codexe2x80x9d, yet uses less tracks than that used with a conventional Gray code encoder. The encoder also has a reduced number of transitions than that required on the track used with a conventional Gray code encoder.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming on or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
The present invention provides an apparatus and method for determining the absolute position of a target. An absolute position sensor to determine the position of a target includes a first line of magnets having a first predetermined number of magnets and a second line of magnets aligned with the first line of magnets having a second predetermined number of magnets. The second predetermined number of magnets is different from the first predetermined number of magnets. A sensor for determining a position on each magnet of the first line of magnets is included. A sensor for determining a position on each magnet of the second line of magnets is also included. A circuit is provided for comparing the position on each magnet of the first line of magnets and the position on each magnet of the second line of magnets to determine an absolute position of the first line of magnets. By comparing the relative positions on the two lines of magnets, the position on the entire line of magnets can be determined by the amount of offset in the positioning of the magnets from each line.
The present invention also provides a method for determining an absolute position of a target. The method of the present invention includes providing a first line of magnets having a first predetermined number of magnets and a second line of magnets aligned with the first line of magnets. The second line of magnets has a second predetermined number of magnets different from the first predetermined number of magnets for the first line. A position on each magnet of the first line of magnets is determined. A position on each magnet of the second line of magnets is determined. The position on each magnet of the first line of magnets is compared with the position on each magnet of the second line of magnets to determine an absolute position of the first line of magnets.